Digital display systems typically produce or modulate light as a linear function of input image data for each pixel. For an 8-bit monochromatic image data word, the input image data word ranges from 0 to 255. A value of 0 results in no light being transmitted to or produced by a pixel, 255 is the maximum intensity level for a pixel, and 128 is mid-scale light.
Pulse width modulation (PWM) schemes typically modulate a constant intensity light source in time periods whose length depends on time weighting of display bits. For example, when 5500 microseconds is available for each color of a three-color system the pixel on times for a system using 9 display data bits might be 14 microseconds, 28 microseconds, 56 microseconds, 112 microseconds, 224 microseconds, 448 microseconds, 896 microseconds, 1861 microseconds, and 1861 microseconds, respectively. If a given display data bit for a particular pixel is a logic 0, no light is emitted from or generated by the pixel. If the display data bit is a logic 1, then the maximum amount of light is emitted from or generated by the pixel during the display data bit period. The viewer's eye integrates the light received by a particular pixel during an entire frame period to produce the perception of an intermediate intensity level.
One problem encountered by PWM display systems is the creation of visual artifacts that arise due to the generation of an image as a series of discrete bursts of light. While stationary viewers perceive stationary objects as having the correct intensity, motion of the viewer's eye or motion in the image can create an artifact of false contours produced by saccadic eye movement, known as PWM temporal contouring. PWM temporal artifacts are described in U.S. Pat. No. 5,619,228. PWM temporal artifacts are created when the distribution of radiant energy is not constant over an entire frame period and may be noticeable when there is motion in a scene or when the eye moves across a scene.
When the eye moves across a scene, a given point on the retina of the eye accumulates light from more than one image pixel during the eye's integration period. If the various pixels are all displaying the same intensity in the same way—the discrete bursts of light are occurring simultaneously for all pixels—the perceived pixel intensity will be correct. If the various pixels are not displaying the same intensity in the same way the eye may register false perceived intensities which may appear brighter or darker than the average displayed intensity. This happens when the discrete bright periods of a first pixel are created during a first portion of the frame period and the eye then scans to a second pixel that uses the next portion of the frame period to display the light. Since the same point on the retina receives the light from the first pixel and the second pixel in rapid succession—less than the decay period of the eye—that point of the retina perceives a single pixel as bright or as dark as the sum of the first and second pixels. This PWM temporal contouring artifact appears as a noticeable pulsation in the image pixels. This pulsation is time-varying and creates apparent contours in an image that do not exist in the input image data.
When viewed at a normal viewing distance, the PWM contouring artifact created by two adjacent pixels is very difficult, if not impossible, for the typical viewer to detect. In real images, however, the bit transitions often occur in areas having a large number of adjacent pixels with virtually identical image data values. If these large areas of similar pixels have clusters whose intensity values cross a major bit transition, the PWM contouring is much easier to detect.
Given the quantization and temporal artifacts created by PWM displays, a process and system of producing very small intensity changes and eliminating noticeable temporal artifacts is needed.
Related subject matter may also be found in U.S. Pat. No. 7,075,506, the entirety of which are incorporated herein by reference.